This invention relates to spectral analysis and more particularly to a high resolution spectrum analyzer for use in anti-submarine warfare applications.
One of the most pressing problems in anti-submarine warfare (ASW) applications is the detection of the spectral signature, direction and velocity of both surface and subsurface vessels by either passive or active listening devices located at sonobuoys which are air dropped across a given area suspected of having enemy vessels.
The purpose of the passive sonobuoys is to listen for subsurface or surface activity and transmit detected signals, usually in the acoustic range, to overflying aircraft. The system to be described is useable with both LOFAR and DIFAR buoys in which the LOFAR buoy is an omnidirectional listening type buoy and the DIFAR buoy includes direction determining apparatus so as to be able to locate the direction of the incoming signals.
The signals from the sonobuoys are, in general, transmitted to the overflying aircraft which identifies the particular sonobuoy from which a signal is coming, its position, and the information carried on the signal. The incoming signals from the sonobuoys are usually analyzed in detail as to the particular signature of the sound source. This includes analysis as to the particular frequencies which are present in the incoming signal, usually in a range from 2.5 Hz to 3,000 Hz; any doppler shift of the spectral lines of the signal; the line width of the spectra so as to identify whether the particular vessel is driven by an internal combustion engine (broad spectral line) or by turbine or electric power (narrow spectral line); the amplitude of each of the spectra of the signal; and the direction of the particular incoming signal, so as to identify the source of either continuous signals or periodic signals indicative of known types of vessels.
In a typical passive listening system, the signals from the sonobuoy are sampled and digitized so as to produce a digital representation of the incoming signal. In order to perform a spectrum, analysis, the digital representation of the incoming signal is multiplied with a digital representation of a reference signal of a predetermined frequency, with the product thereof being integrated to obtain the degree of correlation between the reference signal and the incoming signal. The reference signal is periodically changed or stepped in a known and rapid fashion so that the incoming sampled signal is compared with a large number of reference signals, each at a different frequency. When correlation occurs, the reference frequency is noted as well as the amplitude of the signal. The notation is usually in the form of a xe2x80x9cgramxe2x80x9d type display which gives a time history of the frequency and amplitude of the incoming signal. From this type display, spectral signature information as well as any doppler frequency shift indicating a relative velocity with respect to the particular sonobuoy can be readily observed. The same type information can be obtained from a so-called xe2x80x9cwaterfallxe2x80x9d display or even from the traditional display of the spectra of the incoming in terms of frequency versus amplitude.
The system described utilizes a narrow band digital filter, the center frequency of which is rapidly stepped so that a spectral analysis over a range of frequencies takes place in real time. The system differs generically from Fast Fourier Transform (FFT) analysis as follows: FFT analysis reduces the number of operations to be carried out in performing a spectral analysis while at the same time accepting whatever complexity is involved for the individual operations. The narrow band digital filter reduces the complexity of the individual operations while accepting whatever number of operations naturally arises.
This process is equivalent to passing the signal through a narrow band filter whose band width is equal to the reciprocal of the time epoch over which the cross correlation is performed. By storing a sequence of digitized signal samples (which we call a xe2x80x9crecordxe2x80x9d) in an electronic memory, the samples may be sequenced through the cross correlator at many times the real-time rate. Each time the sequence is recirculated, the reference frequency is stepped to a different frequency thereby accomplishing real time spectrum analysis.
One of the problems with prior ASW spectrum analyzers has been the sensitivity of the spectrum analysis, especially at the relatively low acoustic frequencies which typify the types of radiation from a moving target vessel such as a submarine. The sensitivity of the spectrum analyzer in general is directly proportional to the length of the record of the incoming signals which is processed. The longer the record the better able will be the apparatus to resolve stable signals by virtue of long integration times. Moreover, in terms of bandwidth (BW), bandwidth is inversely proportional to the length of a record processed in a coherent fashion. This means that the longer the coherent record processed the narrower the bandwidth and the greater, the resolution.
The criticality of the resolution of a given spectrum analyzer for use in ASW applications can be seen in a comparison between prior art systems and the present system. It is a feature of the prior art systems that prior art apparatus do not detect doppler shifts of under 2.5 Hz at 1,000 Hz. This doppler shift corresponds to a velocity of the source relative to the sonobuoy of 7.4 knots. Thus, present day apparatus does not detect movement of targets which are proceeding at a speed of under 7.4 knots. This is because in general, present spectrum analyzers utilized in ASW applications have a resolution of xc2x1xc2xc% of the reference frequency. On the other hand, the subject system to be described permits resolution of xc2x1{fraction (1/64)}% of the reference frequency. This results in a resolution at 1,000 Hz. of 0.156 Hz which is equivalent to 0.46 knots. Thus, it is virtually impossible for a target vessel to escape a doppler measurement by virtue of moving slowly through the water.
The subject system is virtually unlimited in resolution by virtue of performing the spectrum analysis with a number of short, albeit discontinuous, records of the sampled signal which can be built up to any desired length and which can be made to simulate continuous records. The ability to use discontinuous records permits batch processing such that while one short record is being correlated with a large number of reference signals each having a different frequency, a second record is being built up in the memory of the system to form another short record. With the subject invention it is possible to process data blocks separately and add them up so that the effective length of the record may be increased, for instance, by an order of magnitude without a corresponding increase either in the physical size of the memory or a corresponding increase in the amount of processing equipment.
Virtually unlimited resolution is accomplished since if T= record length, the resolution of the system is 1/T For example, if a record length at 100 Hz=2000 samples, then T=4 seconds and the resolution or bandwidth BW=xc2xc Hz. To increase the resolution, the record length is increased. For a resolution of {fraction (1/32)} Hz the record length is multiplied by 8 so as to increase the number of samples from 2,000 samples to 16,000 samples. To accumulate 16,000 samples in one register and to process them would require a large amount of additional storage and processing.
In the subject invention, to solve the problem of the amount of storage and processing normally necessary, spectrum analysis is performed by breaking up the record into subrecords or segments, by processing the information in the subrecords or segments so as to eliminate the effects of the discontinuities and then by accumulating the results in accordance with the following equation for the above example:       E    o    i    =                    ∑                  i          =          1                          i          =                      16            ,            000                              ⁢              xe2x80x83            ⁢                        S          i                ⁢                  R          i                      =                            ∑                      i            =            1                                i            =            2000                          ⁢                  xe2x80x83                ⁢                              S            i                    ⁢                      R            i                              +                        ∑                      i            =            2001                                i            =            4000                          ⁢                  xe2x80x83                ⁢                              S            i                    ⁢                      R            i                              +              …        ⁢                  xe2x80x83                ⁢                              ∑                          i              =              14001                                      i              =                              16                ,                000                                              ⁢                      xe2x80x83                    ⁢                                    S              i                        ⁢                          R              i                                          
Here Si=input signal sample and Ri=reference signal sample.
The above block processing and accumulation can be accomplished if, and only if, the records which are initially discontinuous by virtue of being sectorized are effectively made continuous. This is to say that as a necessary condition there must be phase coherence between the reference signals utilized and those represented by the processed records. To establish phase coherency, the starting phase of the reference generator is adjusted for the length of the input signal which is lost due to the discontinuity between adjacent records and for the particular reference frequency employed.
Once the reference signal starting phase is adjusted to be that which is expected of a stable input signal which continues through the period of discontinuity, then phase coherency is effectively established and the batch processing may proceed. In the subject system, this is accomplished as follows:
A high resolution spectrum analyzer is provided which utilizes a frequency-stepped narrow band digital filter. To duplicate the long record, an incoming signal is divided up into short discontinuous records which represent discontinuous portions of the input signal. Each of these discontinous portions is correlated in a correlator which multiplies the incoming signal as represented by the discontinuous record with a large number of reference signals, each at a different frequency. In the correlation, this product is integrated to give the degree of correlation between the particular segment of the incoming signal and a particular reference signal. The results of the processing of each short signal record at a given frequency are accumulated and added together, one frequency and then another etc., with the results being the correlations of the input signal with the reference signals over a record having an effective length equal to the sum of the lengths of the short records. Any lack of phase coherency between the reference and input signals due to the discontinuity of the records is compensated for by measuring the length of input signal lost due to the discontinuity between adjacent input signal records and adjusting the starting phase of the reference signal at the start of the later record in accordance with the length of input signal lost and in accordance with the frequency of the reference signal so that the starting phase of the reference signal is equal to the phase of the input signal which would be expected to exist at the end of the discontinuity, were there no discontinuity.
Although batch processing necessitates splitting up of the record and processing different batches of data in a discontinuous manner, when high density rotating storage drums are used further problems are introduced. One of the problems is that data is typically written onto the drum in sectors or segments which are to carry a predetermined number of signal samples. However, due to variation or dither of the drum speed, and the asynchronous relationship between the drum and system clocks, gaps are created between successive signal records which gaps result in discontinuities in which varying numbers of data samples are unprocessed or dropped. Thus, with the variation in drum speed there are differing number of samples dropped and thus different lengths or discontinuities.
The solution to both batch processing discontinuities and the variable length discontinuities is to count the number of dropped samples and adjust the starting phase accordingly, since there is a fixed relationship between the change in the starting phase, xcfx86, missed sample count, h, sampling rate, Fs, and reference frequency, fi.
Having determined the necessary phase adjustment for coherent spectrum analysis of the next record, the phase of the reference signal generator is adjusted via a phase selector and the output signal from the generator is coupled to the correlator. It will be appreciated that the referenced generator, phase selector, and correlator form a narrow band filter.
In one embodiment of the subject invention a so-called alpha (xcex1) generator is used to generate digital representations of the reference signals. The frequency, fr, of the xcex1 generator is determined by the xcex1, a binary code, which forms one of the inputs to the generator. In general, the output of the xcex1 generator is a series of binary coded signals, each of which represent a phase angle. These phase angles are then sequentially converted to amplitudes to form the reference signal.
Prior to the generation of the reference signal, the phase of the output of the xcex1 generator is set by making the generator""s output code correspond to the desired starting phase, in this case, xcfx86. Thus, the number of missed samples is computed, and a binary phase angle input code, xcfx86, similar to xcex1 is generated which is dependent both on the missed sample count and the desired xcex1. This signal is applied to the xcex1 generator to initially set the phase of the signal to be generated. The xcfx86 signal is applied to the xcex1 generator one clock pulse prior to the application of the xcex1 signal corresponding to the particular frequency of interest. The setting of the starting phase is easily accomplished by passing the binary phase angle input code directly to the output terminals of the alpha generator, since the output from the alpha generator is normally a series of binary output codes each of which represents a phase angle. After setting the first output code, the next series of output codes are then sequentially converted to amplitudes to generate a reference signal having the required starting phase
It is therefore an object of this invention to provide an improved spectrum analyzer capable of processing discontinuous records.
It is another object of this invention to provide an improved method and apparatus for detection of surface or subsurface vessels.
It is still a further object of this invention to improve the resolution of the spectral analysis in the acoustic frequency range by more than an order of magnitude through increasing of the length of record processed in a cross correlation type spectrum analyzer.
It is a still further object of this invention to provide coherency between discontinuous records in which the length of the discontinuity between adjacent records is measured and in which the starting phase of a reference signal is adjusted in accordance with the amount of signal lost over the discontinuity and in accordance with the particular reference frequency utilized.
It is a still further object of this invention to solve the problem of variable drum speed when rotating drum type memories are utilized to store incoming data, such that coherent cross correlation may be accomplished over what is effectively an extremely long record, or intergration time.
These and other objects of the subject invention will be better understood in connection with the following detailed description and the drawings in which: